The present invention relates to xerographic process control, and more particularly, to the use of data from prior printed customer images to provide toner control patches that are relatively unaffected by reload.
Many printing devices use donor rolls to transfer toner to the surface of a photoreceptor for developing an image thereon. These donor rolls typically accumulate toner as they rotate. After transferring toner to an image or a portion of an image, the donor roll xe2x80x9creloadsxe2x80x9d with toner as it rotates. Depending on what was imaged before the image or portion of an image being developed, the donor roll may not be able to accumulate a sufficient level of toner to properly develop the current image. This inability to fully reload the donor roll causes the later drawn image or portion of an image to have an area lighter than it should be.
In particular, hybrid scavengeless development (HSD) systems use a magnetic brush of a conventional two component system in conjunction with a donor roll used in typical single component systems to transfer toner from the magnetic brush to the photoreceptor surface. Consequently, the donor roll must be completely reloaded with toner in just one revolution. The inability to complete reloading of the donor roll in one revolution results in a print quality defect called reload error. Reload error is defined by a depletion of toner on the donor roll of the development housing.
It should be noted that reload error can occur in any device using a donor roll, where the donor roll needs to be completely reloaded in one revolution, and is not limited to HSD systems.
One example of this defect occurs where the structure of an image from one revolution of the donor roll is visible in the image printed by the donor roll on its next revolution, a phenomenon known as ghosting in the art related to single component xerographic development. At locations on the donor roll where previous images were located, the level of toner may be lower than desired. This causes an undesirable lightening of parts of an image, depending on what was imaged earlier. Highly conductive developers aid in the reduction of this defect as developers that are more conductive allow for a more maximal transfer of toner from the magnetic brush to the donor roll. However, reload error problems can still remain.
Adjusting the xerographic parameters can significantly reduce reload error, but this can also lead to high levels of mottle, another image quality problem characterized by the non-uniform printing or coloring of an image. Thus this method for correcting the reload error problem leads to a trade-off between reload error and mottle, and in order to prevent unacceptable reload error problems it is necessary to tolerate a higher level of mottle.
One area where reload error may have a significant effect is in color calibration systems. In copying or printing systems, such as a xerographic copier, laser printer, or ink-jet printer, a common technique for monitoring the quality of prints is to artificially create a xe2x80x9ctest patchxe2x80x9d of a predetermined desired density. The actual density of the printing material (toner or ink) in the test patch can then be optically measured to determine the effectiveness of the printing process in placing this printing material on the print sheet.
In the case of xerographic devices, such as a laser printer, the surface that is typically of most interest in determining the density of printing material thereon is the charge-retentive surface or photoreceptor, on which the electrostatic latent image is formed and subsequently, developed by causing toner particles to adhere to areas thereof that are charged in a particular way. In such a case, the optical device for determining the density of toner on the test patch, which is often referred to as a xe2x80x9cdensitometerxe2x80x9d, is disposed along the path of the photoreceptor, directly downstream of the development unit. There is typically a routine within the operating system of the printer to periodically create test patches of a desired density at predetermined locations on the photoreceptor by deliberately causing the exposure system thereof to charge or discharge as necessary the surface at the location to a predetermined extent.
The test patch is then moved past the developer unit and the toner particles within the development unit are caused to adhere to the test patch electrostatically. The denser the toner on the test patch, the darker it will appear in optical testing. The developed test patch is moved past a densitometer disposed along the path of the photoreceptor, and the light absorption of the test patch is tested; the more light that is absorbed by the test patch, the denser the toner on the test patch. Some toner mass sensors also measure the light scattered by test patches in addition to or instead of measuring the light absorbed by the patches to arrive at a toner mass for the patch.
Xerographic test patches are traditionally printed in the interdocument zones on the photoreceptor. They are used to measure the deposition of toner on paper to measure and control the tone reproduction curve (TRC). Generally, each patch is printed as a uniform solid half tone or background area. This practice enables the sensor to read one value on the tone reproduction curve for each test patch.
The traditional method of process controls involves scheduling solid area, uniform halftones or background in a test patch. Some of the high quality printers contain many test patches. During the print run, each test patch is scheduled to have single halftone that would represent a single byte value on the tone reproduction curve. For example, U.S. Pat. No. 5,060,013 discloses a control system using test patches at different locations within the image frame on the photoreceptor. A plurality of sensors is arranged to sample the test areas in defined columns of the frame and measurements coordinated with the location of the test area.
It is also known in the prior art, for example, U.S. Pat. No. 4,341,461 to provide two test targets, each having two test patches, selectably exposed to provide test data in the photoreceptor image area for control of the toner dispensing and bias control loops. In this system, the test patches are imaged in interdocument zones on the photoreceptor. In addition, U.S. Pat. No. 5,450,165 discloses the use of incoming data or customer image data as a test patch. In particular, incoming data is polled for preselected density conditions to be used for test patches to monitor print quality.
It is also known, in pending U.S. Pat. No. 5,543,896, to provide a single test pattern, having a scale of pixel values, in the interdocument zone of the imaging surface and to be able to respond to the sensing of the test pattern and a reference tone reproduction curve to adjust the machine operation for print quality.
Embodiments include a toner control process, which includes substantially determining how reload error will affect a printed image, modulating the color density of a test patch to compensate for reload error prior to printing the test patch, printing the modulated test patch, sensing the digital image, adjusting toner output according to the sensed digital image.